This application claims the Paris convention priorities of Japanese Patent Applications Nos. 8-46575 and 8-46576 both filed on Feb. 7, 1996.
It is known to use a sintered nickel electrode as the nickel electrode of an alkaline storage battery such as an Ni--Cd battery or a nickel-hydrogen battery. For the purpose of making this electrode, it is well known that nickel powder is sintered on the surfaces of a substrate made of a porous steel plate and then this substrate is impregnated with an active material (e.g. nickel hydroxide).
In order to improve the bulk density of an active material used in a sintered nickel electrode, it is necessary to use a highly porous sintered substrate. However, a highly porous sintered substrate has a disadvantage that it is apt to allow the nickel powder to fall off therefrom because of a weak bond between nickel grains resulting from sintering. In order to prevent the nickel powder from falling off from the highly porous sintered substrate, it is most common to make the porosity of the sintered substrate lower than 80%. However, the porosity of the sintered substrate lower than 80% prevents the active material from having a bulk density which is large enough to be adapted for use in a sintered nickel electrode. The highly porous sintered substrate has another disadvantage that a cumbersome process of impregnation has to be repeated several times by an infiltration method for filling the substrate (i.e. a sintered compact) with the active material. This is because a sintered compact obtained from nickel powder usually has a pore size smaller than 10 .mu.m.
A non-sintered nickel electrode has been recently proposed for the purpose of eliminating these disadvantages, where the electrode is made by directly filling a highly porous substrate (such as a foamed metal plated with an alkali-proof metal) with an active material (e.g. nickel hydroxide) fluxed with a solution of a binding agent (e.g. an aqueous solution of methyl cellulose). Since a highly porous substrate (having a porosity of 95% or more) can be used for the non-sintered nickel electrode, the active material is allowed to have a large bulk density and only a single process is enough to fill the substrate with the active material.
However, the highly porous substrate used for the improved bulk density of the active material in the non-sintered nickel electrode is accompanied by a lower electric conductivity and a lower availability of the active material resulting from a lower current collecting capacity than a sintered substrate used in a sintered nickel electrode.
For the purpose of improving the electric conductivity of the non-sintered nickel electrode, it has been proposed that cobalt hydroxide powder (as disclosed in Japanese Laid Open Patent Application No. 61-74261) or graphite powder (as disclosed in Japanese Laid Open Patent Application No. 7-211316) should be added to nickel hydroxide powder.
However, a study made by the present inventors has revealed that, even if cobalt hydroxide powder or graphite powder is added to an active material, it is difficult to obtain a non-sintered nickel electrode in which the utilization of active material is sufficiently high, i.e. in which the active material is made available in a sufficiently high degree.
For the purpose of improving the electric conductivity of the non-sintered nickel electrode, it has also been proposed that the active material should take the form of composite particles consisting of grains of nickel hydroxide coated with cobalt hydroxide (as disclosed in Japanese Laid Open Patent Application No. 62-234867) or those consisting of grains mainly constituted by nickel hydroxide and coated with cobalt oxyhydroxide (as disclosed in Japanese Laid Open Patent Application No. 3-78965).
However, another study made by inventors has revealed that, even with such an improvement in the active material, it is difficult to obtain a non-sintered nickel electrode in which the utilization of active material is sufficiently high.